Aircrafts employ altimeter systems to determine the altitude of the aircraft above terrain. Typical frequency modulation-continuous wave (FM/CW) radar altimeter systems use a single servo loop that attempts to detect a leading edge of a single first return and adjust the parameters of the radar altimeter modulation so as to maintain a constant input frequency to signal processing equipment of the altimeter. In traditional systems only one potential altitude detection is made, ignoring the possibility that the earliest reply is not the ground but some other intervening object such as a tree top, building top, another plane, large machinery top such as construction cranes, rain, etc. This causes the altimeter to display altitudes that are too low and sometimes dangerously too high.
Current altimeter designs also attempt to smooth output altitude by averaging many sequential altitude measurements. Unfortunately this method permits one or more erroneous or large change measurements to skew the final computed value. In particular, prior art altitude tracking schemes revolve around “instantaneous detection” of the “leading edge” or “mean of the return spectrum” of return radar signals on a modulation to modulation period basis. Typically for each modulation period, the computed altitude is averaged with the succeeding period of N periods. This tends to cause any “outlying data points” to pull or skew the computed result. Many existing altitude tracking schemes utilize a suite of system loops that adjust intermediate frequency (IF) automatic gain control (AGC) and the modulation rate which in turn causes the altitude resolution of the altimeter to vary inversely to the altitude—(i.e. less resolution at greater altitude) in addition to the basic altitude tracking loop.
Moreover, existing radar altimeters are frequently based on FM/CM or pulsed modulation. These altimeters compute only a limited altitude extent of range gates centered around where the altimeter is either seeking or tracking ground reflections. These limited altitude extents can mislead the tracking algorithms or incur delays in acquiring or following rapidly changing topology. The tracking loop may be adversely impacted if the rate of change of altitude is greater than the tracking window altitude extent. Algorithms have been demonstrated to indicate greater or lesser altitude than in actually the case because of the need to adjust the limited signal processing extent over constantly changing reflection amplitude and complex ground structures.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for Radar Altimeters that are immune to the effects of a) step change in ground altitude, b) brightness variation in ground reflections, c) rain echo effects, d) aircraft below own aircraft and E) snow/rain/dust covered runways.